System and method for locating earth fault in power grids

ABSTRACT

In a multi-phase power grid fed by a power source, earth fault (460) is located by means of a power supply source synchronized with the power grid, which is connected between a zero point of the grid and earth. In a fault current compensation mode (420), a control unit controls the alternating voltage source to compensate for any ground fault current in the power grid to a value below a threshold level. In a fault detecting mode (430), the control unit gradually adjusts the output voltage of the alternating voltage source with respect to amplitude and/or phase angle (440). A change of zero-sequence current and zero-sequence admittance between the alternating voltage source and a fault location is measured (450) by means of at least one detector. The at least one detector is communicatively connected to the control unit and reports recorded measured values representing zero-sequence current and/or zero-sequence admittance to the control unit. In the fault detecting mode, the control unit localizes a ground fault (460) based on at least one of said measurement values representing changes of the zero-sequence current and/or zero-sequence admittance, upon which an affected branch is disconnected (470) or the system switches to the fault compensation mode (420).

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention generally relates to solutions for compensatingearth fault currents in a multiphase power grid. In particular, theinvention relates to a system for locating earth fault according to thepreamble of claim 1 and a corresponding method. The invention alsorelates to a computer program and a process-readable medium.

There are today technical solutions to quickly and fully compensate fora fault current at earth fault in a high voltage grid. For example, theapplicant has designed a system which can eliminate any error current inless than 60 mils without influencing the power transfer by themalfunctioning device part. Such a quick intervention is of courseadvantageous as it significantly reduces the risk of consequentialdamage, such as short circuits, burns and/or personal injuries.

WO 2014/021773 discloses a solution in which a controllable groundingtransducer is arranged to compensate for a residual operating current ina grounding fault of an alternating voltage power supply network with apower supply transformer. A primary winding of the grounding transformeris coupled to the power grid and a secondary coil of the groundingtransformer is coupled between a zero point of the power grid andground, the grounding transformer comprising two or more windingcouplers and a control unit which via the winding couplers controls thesecondary voltages of the grounding transformer with amplitudes andphase angles relative to the voltage signal of the power supplytransformer.

Problem with Previously Known Techniques

One disadvantage of a very fast intervention is that the possibility oflocating the actual fault location is severely limited. Because forregulatory reasons, it is often required that a possible fault currentcan be rapidly reduced to zero, or near zero, there is still no methodavailable for locating the fault location caused the error current. Forexample, the fault current should not exceed a threshold value which istypically significantly lower than the current that occurs if supplyvoltage would be applied to detect a possible error. The post locatingthus risks causing spark formation which in turn may cause, for example,forest fire. Therefore, with the methods known to date, it is impossibleto determine in a safe way whether the error was de facto transient, asis often the case in overhead line network, or if the error is ofpermanent nature.

SUMMARY OF THE INVENTION

The object of the invention is therefore to solve the above-mentionedproblem, thus providing a means of locating any earth fault in amultiphase power grid while satisfying the current requirements bysolving the elimination of fault currents in a reliable and rapidmanner.

According to one aspect of the invention, the object of the initiallydescribed system is achieved, the system comprising at least onedetector arranged in the power grid and communicatively connected to thecontrol unit. The at least one detector is capable of registering themeasurement values representing zero-sequence current and zero-sequenceadmittance. In a fault detection mode, the control unit is configured togradually adjust an output voltage from the alternating voltage sourcewith respect to amplitude and/or phase angle so that one of the changedzero-sequence current and zero-sequence admittance between thealternating voltage source and a fault location may be measured by theat least one detector. The at least one detector is in turn configuredto apply registered measured values representing zero-sequence currentand/or zero-sequence admittance to the control unit. In the faultdetection mode, the control unit is further configured to detect aground fault based at least on the measurement values representingchanges of the zero-sequence current and/or zero-sequence admittance.

This system is advantageous because it makes it possible to find a faultlocation without risking exceeding a maximum power current at the faultlocation. It is unproblematic to allow the control unit to adjust thevoltage of the alternating voltage source so that its output current inthe fault detection mode is lower than a certain threshold, say 0.5 A.This current is the limit of post-localization of earth fault in theareas of Australia that is particularly sensitive to forest or grassfires.

According to a preferred embodiment of this aspect of the invention, thecontrol unit is configured to switch to the fault compensation modeafter fault detection mode. Consequently, power transmission via themalfunctioning device part can continue until the exact location of thefault location has been established and the necessary resources forrepairing the fault have been developed.

According to another preferred embodiment of this aspect of theinvention, the control unit, in the fault detection mode, is configuredto control the alternating voltage source to output an alternatingvoltage in the power grid, which alternating voltage is superposed thevoltage of the power grid and is gradually changing and whose frequencydiffers from a frequency of the power grid. This facilitates thedetection of earth fault through the at least one detector.

It is particularly preferred if the control unit is configured tocontrol the alternating voltage source to output an alternating voltagewith a specific superimposed signal pattern and if the at least onedetector is configured to detect the specific signal pattern. Thus, sometypes of earth faults can be detected and localized even more effective.

According to another aspect of the invention, the object of theabove-described method is achieved, wherein measured values will beregistered that represent zero-sequence current and zero-sequenceadmittance by means of at least one detector arranged in the power grid,and in a fault detection mode. A voltage output from the alternatingvoltage source is gradually adjusted with respect to amplitude and/orphase angle so that a change of zero-sequence current and zero-sequenceadmittance between the alternating voltage source and a fault can bemeasured using the at least one detector. On the basis of one of themeasured values representing a change of zero-sequence current and/orzero-sequence admittance, a possible earth fault is detected. Theadvantages achieved by this method, as well as with the preferredembodiments thereof, are apparent from the discussion above withreference to the proposed system.

According to a further aspect of the invention, the object is achievedby a software program which is loadable to the memory of at least oneprocessor, wherein the computer program comprises software for executingthe above suggested method when the computer program is running in theat least one processor.

According to another aspect of the invention, the object is achieved bya computer readable medium having a program stored therein, the programbeing configured to cause at least one processor to execute the abovesuggested method when the program is loaded in the at least oneprocessor.

Further advantages, advantageous features and applications of thepresent invention will be apparent from the following description andthe dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail by means ofembodiments, which are shown by way of example, with reference to theaccompanying drawings.

FIG. 1 illustrates a first prior art solution for resonance grounding.

FIG. 2 illustrates a second prior art residual current compensationsolution.

FIG. 3 shows a single-line diagram according to one embodiment of theinvention for locating earth fault.

FIG. 4 illustrates, by means of a flowchart, a method according to anembodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Initially, reference is made to FIG. 1, which illustrates afirst-mentioned known solution for so-called resonance grounding in amulti-phase power grid. FIG. 1 shows a transformation of the power gridto its symmetric component. The power grid here includes a power source110 which supplies a driving voltage E to the power grid and sourceimpedances Z₊, Z⁻ and Z₀ and also a payload Z_(L) in the power grid.

If earth fault occurs in the form of a single phase grounding 1Ø thefault current can be reduced by means of a zero-point reactor 120connected between the zero point N and the ground E. The zero-pointreactor 120 is a variable inductance L which forms a parallel resonancecircuit with the capacitance leakage current 3C₀ of the power grid. Onthe one hand, transfer of the payload Z_(L) must be limited to a plus-and minus-sequence system, that is, between the faults; and on the otherhand, the zero-point reactor 120 must be capable of matching the varyingleakage currents that occur in the power grid during operation.

The basic prerequisite for limiting the power transmission to the plussequence and the minus sequence is given in the vast majority ofexisting power grids. Resonance grounding is today the predominantsystem of the existing high-voltage distribution networks.

An immediate effect of current limitation at single phase landings 1Ø isthat light arc overlays, which are the most frequent errors in overheadline network, are self-sealed. The zero-point reactor 120 is thereforealso called extinguishing coil, or Petersen coil after inventor WaldemarPetersen.

At ground fault, zero-point reactor 120 compensates for the capacitiveleakage currents. The resistive leakage currents remain withoutcompensation, and the resistive leakage currents usually represent 5-10%of the total earth fault current.

As more and more overhead line networks are replaced by buried cables,the capacitive leakage currents of the power grid generally increase.This also increases resistive leakage currents in power grids. As aconsequence, the uncompensated resistive residual currents alsoincrease, which in turn risks exposing the self-extinguishing functionin that part of the power grid that still includes overhead linenetworks. For security reasons, this is of course unacceptable.

FIG. 2 illustrates a second prior art solution for residual currentcompensation, which is a further development of the structure of FIG. 1.The now tuned parallel resonance circuit 3C₀//L has here been omittedfor the sake of clarity.

In analogy with FIG. 1, FIG. 2 shows a driving voltage E from a powersource 210, source impedances Z₊, Z⁻ and Z₀ and a payload Z_(L). Inaddition, a residual current compensation device 220 is included, whichin turn includes a voltage source (−) E which is synchronized with thepower grid, which injects a compensation current between the zero pointand ground of the power grid, which compensating current is equal to theresidual current, but phase distorted 180° relative to a phase angle ofthe residual current. The voltage source (−)E of the residual currentcompensation device 220 is parallel to the sum of the resistive currentsR₀/3 of the power grid. Then, as mentioned above, the known solutions donot allow a localization of a fault location during residual currentcompensation while meeting the regulatory requirements for maximumcurrent strength, the invention aims at solving this problem.

FIG. 3 shows a single-line diagram according to one embodiment of theinvention for locating earth fault in a multi-phase power grid.

In general terms, the proposed methodology assumes that once an errorhas been detected and the earth fault current has been compensatedaccording to the above described with reference to FIG. 2, thevoltage/current of the assumed fault location gradually increases whileappropriate parameters are measured in the power grid. In this faultdetection mode, the same alternating voltage source is used, which isused for residual current compensation in a fault current compensationmode.

More specifically, in the fault detection mode, a fault localizationsignal is superimposed by means of the power supply (−)E for residualcurrent compensation. A relationship between current and voltage in thefault location is determined by an initially unknown fault impedance,together with a source impedance of a fault circuit. In order to ensurethat the current resulting from the localization signal does not exceedgiven limits, the voltage of the localization signal is graduallyincreased until the fault location is determined by means ofspecifically adapted detectors, alternatively until a maximum voltagelevel has been achieved, whichever occurs first.

In FIG. 3, the multiphase power grid is fed by a power source 310. Thesystem proposed according to the invention comprises a power gridsynchronized alternating voltage source 380 which is connected between azero point N of the grid and ground E.

The system also includes a control unit 370 which is capable ofcontrolling the alternating voltage source 380 in a fault currentcompensation mode to compensate for any ground fault current ΔI in aresonance grounded power grid to a value underlying a threshold level.Further, a three-phase measurement transformer 340 is connected to thecontroller 370, which three-phase measurement transformer 340 isconfigured to measure a zero-sequence voltage 3U₀ to determine if groundfault exists in the power grid.

The system further includes at least one detector, here exemplified by351, 352, 35N, 361, 362 and 36N, which detector is arranged in the powergrid and communicatively connected to the control unit 370. The at leastone detector 351, 352 35N, 361, 362 and/or 36N are further configured toregister the measurement values Y_(OL1P), Y_(OL1D), Y_(OL2P), Y_(OL2D),Y_(OLNP), and Y_(OLND) representing zero-sequence current andzero-sequence admittance, so that these parameters can be reported tothe control unit 370.

During current residual current compensation, the fault detection modecan be activated automatically, or in response to a command to thecontrol unit 370, which command has been generated by an operator of thepower grid.

In the fault detection mode, the control unit 370 is configured toprogressively adjust an output voltage Up from the alternating voltagesource 380 with respect to amplitude and/or phase angle so that azero-sequence current and/or a zero-sequence admittance betweenalternating voltage source 370 and a possible fault location changes.

The at least one of the detectors 351, 352, 35N, 361, 362 and/or 36Nregisters the measurement values Y_(OL1P), Y_(OL1D), Y_(OL2P), Y_(OL2D),Y_(OLNP) and Y_(OLND) representing zero-sequence current and/orzero-sequence admittance and transfers these measurement valuesY_(OL1P), Y_(OL1D), Y_(OL2P), Y_(OL2D), Y_(OLNP), and Y_(OLND) to thecontrol unit 370.

In the fault detection mode, the control unit 370 is configured tolocalize a ground fault GF based on at least one of the measured valuesY_(OL1P), Y_(OL1D), Y_(OL2P), Y_(OL2D), Y_(OLNP), and Y_(OLND)representing zero-sequence current and/or zero-sequence admittance fromthe at least one detector 351, 352, 35N, 361, 362 and/or 36N. At thesame time, by measuring a current in a grounding 390 of a zero pointreactor 330, the control unit 370 checks that a change of current ΔIdoes not exceed a maximum allowable level.

Preferably, the control unit 370 is configured to switch to faultcompensation mode after the fault detection mode has ended if a groundfault GF has been located. Alternatively, the associated branchconductor line, such as L_(N) in FIG. 3, may be disconnected from thepower source 310. In FIG. 3 is shown current selector 321, 322 and 32N,which are arranged on a respective line L₁, L₂ and L_(N), and areindividually controllable from control unit 370 on the basis of acontrol signal Ctrl_(SW).

If no earth fault is detected, the control unit 370 is preferablyconfigured to disconnect the alternating voltage source 380.

According to a preferred embodiment of the invention, the control unit370 in the fault detection mode is configured to control the alternatingvoltage source 380 to output an alternating voltage to the power grid,which (i) are superimposed the voltage of the power grid, (ii) isgradually changing and (iii) whose frequency differs from a frequency ofthe power grid. Accordingly, detection of the signal is facilitated bythe at least one detector 351, 352, 35N, 361, 362 and/or 36N.

Particularly preferred is whether the control unit 370 is configured tocontrol the alternating voltage source 380 to output an alternatingvoltage to the power grid with an superimposed signal pattern, and theat least one detector 351, 352, 35N, 361, 362 and/or 36N is specificallyconfigured to detect this superimposed signal pattern.

It is generally preferred if the control unit 370 is configured tooperate the above described procedure in a completely automatic manner,for example by executing a computer program in a processor. Therefore,the control unit 370 advantageously includes a memory device 375 whichstores a computer program including software for executing the procedurewhen the program is running in the processor.

In order to summarize, and with reference to the flowchart of FIG. 4, wewill now describe an embodiment of the method of the proposed invention.

In a first step 410, it is examined if a fault current compensation modeis to be applied. If this is not the case, the procedure loops and stopsat step 410. If in step 410 an earth fault has been detected, a step 420follows in which compensation is made for an earth fault current so thatthe earth fault current is below a threshold. Then a step 430 follows.

In step 430, it is checked if a fault detection mode is to be activated.If this is not the case, the procedure loops back to step 420 forcontinued compensation of ground fault current. If at step 430 it isfound that the fault detection mode is to be activated, steps 440 and450 are activated, preferably parallel to each other.

In step 440, the alternating voltage source is controlled to graduallyadjust an output voltage from the alternating voltage source withrespect to amplitude and phase angle so as to result in a zero-sequencecurrent and zero-sequence admittance between the alternating voltagesource and a fault location is changed.

At step 450, a zero-sequence current and/or a zero-sequence admittancebetween the alternating voltage source and the fault location ismeasured by means of specifically adapted detectors mounted in the powergrid.

After step 440, a step 480 follows, where it is checked whether theoutput voltage Up from the alternating voltage source is lower than orequal to a maximum allowed value U_(Pmax). If so, the procedure proceedsto a step 460, and otherwise a step 490 follows.

After step 450 follows a step 460 where it is investigated if a groundfault has been detected by the measurements in step 450. If so, a step470 follows, and otherwise the loop proceeds back to steps 440 and 450for continued fault localization.

At step 470, it is checked if the line branch affected by the groundfault should be disconnected; and if so, disconnection of the affectedline branch occurs, the procedure proceeds to step 410. Otherwise, theprocedure returns to step 420.

At step 490, the alternating voltage source is disconnected. Then theprocedure returns to step 410.

The above described steps, as well as any random sequence thereofdescribed with reference to FIG. 4 can be controlled by a programmedprocessor. In addition, although the above-described embodiments of theinvention, with reference to the figures, comprise a computer andcomputer-implemented processes, the invention extends to particularly,on a carrier or in a carrier, being adapted to practically implement theinvention. The program may be in the form of source code, object code, acode that represents an intermediate between source- and object-code, asin a partially compiled form, or in any other form appropriate to useupon the implementation of the present invention. The carrier may be anyentity or device capable of carrying the program. For example, thecarrier may comprise a storage medium such as a flash memory, a ROM(Read Only Memory), for example, a CD (Compact Disc) or a SemiconductorROM, EPROM (Electrically Programmable ROM), EEPROM (Erasable EPROM), ora magnetic recordable medium, for example, a floppy or hard disk. Inaddition, the carrier may be an overloaded carrier such as an electricalor optical signal which can be passed through an electrical or opticalcable or via radio or by other means. When the program is made up of asignal that can be directed directly by a cable or other device ormember, the carrier may be such a cable, device or member.Alternatively, the carrier may be an integrated circuit in which theprogram is embedded, wherein the integrated circuit is adapted toperform, or to be used in carrying out the relevant processes.

The invention is not limited to the embodiments described with referenceto the figures but can be varied freely within the scope of the appendedclaims.

The term “includes/including” when used herein, this term is understoodto refer to the presence of the specified features, integers, steps orcomponents. However, the term does not exclude the presence or additionof one or more additional features, integers, steps or components, orgroups thereof.

The invention claimed is:
 1. A system for locating earth fault in amulti-phase power grid powered by a power source, the system comprising:an alternating voltage source synchronized with the power grid and whichis connected between a zero point of the power grid and earth; acontroller configured to control the alternating voltage source when ina fault current compensation mode to compensate for an earth faultcurrent to maintain the earth fault current at a value below a thresholdlevel; and at least one detector arranged in the power grid andcommunicatively connected to the controller, the at least one detectorbeing configured to determine measurement values representingzero-sequence current and zero-sequence admittance, wherein thecontroller, when in a fault detection mode, is configured to graduallyadjust a voltage from the alternating voltage source with respect toamplitude and/or phase angle so that a change of zero-sequence currentand zero-sequence admittance between the alternating voltage source anda localization point can be measured by the at least one detector,wherein the at least one detector is configured to send the determinedmeasurement values representing zero-sequence current and/orzero-sequence admittance to the controller, wherein the controller, whenin the fault detection mode, is configured to localize an earth faultbased on at least one of said measurement values representing changes ofzero-sequence current and/or zero-sequence admittance, and wherein thecontroller is configured to switch to the fault current compensationmode after the fault detection mode, wherein the controller isconfigured to operate first in the fault detection mode and then in thefault current compensation mode that follows the fault detection mode.2. The system of claim 1, wherein the controller, when in the faultdetection mode, is configured to control the alternating voltage sourceto output an alternating voltage to the power grid, the alternatingvoltage gradually changing and having a frequency that differs from afrequency of the power grid.
 3. The system of claim 1, wherein thecontroller is configured to control the alternating voltage source tooutput to the power grid an alternating voltage with an superimposedsignal pattern, and the at least one detector is configured to detectthe superimposed signal pattern.
 4. A method of locating earth fault ina multiphase power grid fed by a power source using an alternatingvoltage source that is synchronized to the power grid, and which isconnected between a zero point of the power grid and earth, the methodcomprising: controlling, in a fault current compensation mode of acontroller, the alternating voltage source to compensate for an earthfault current in the power grid to maintain the earth fault current at avalue below a threshold level; determining measurement valuesrepresenting zero-sequence current and zero-sequence admittance using atleast one detector arranged in the power grid; gradually adjusting anoutput voltage of the alternating voltage source with respect toamplitude and/or phase angle, when in a fault detection mode of thecontroller, so that a change of zero-sequence current and zero-sequenceadmittance between the alternating voltage source and a fault locationcan be measured with the at least one detector; localizing an earthfault based on at least one of said measurement values representingchanges of zero-sequence current and/or zero-sequence admittance; andswitching the controller into the fault current compensation mode afterthe fault detection mode, wherein the controller is configured tooperate first in the fault detection mode and then in the fault currentcompensation mode that follows the fault detection mode.
 5. The methodof claim 4 further comprising controlling the alternating voltagesource, when in the fault detection mode, to output an alternatingvoltage to the power grid, is the alternating voltage being graduallychangeable and having a frequency that differs from a frequency of thepower grid.
 6. The method of claim 4, comprising: controlling thealternating voltage source to output an alternating voltage to the powergrid having a superimposed signal pattern, and detecting thesuperimposed signal pattern by means of the at least one detector.
 7. Acomputer program loadable into a memory of at least one processor,including software for executing the method of claim 4, when thecomputer program is running in the at least one processor.
 8. Aprocessor-readable non-transient medium having a program stored therein,wherein the program is arranged to cause at least one processor toexecute the method of claim 4 when the program is loaded into the atleast one processor.